1. Jet Quenching Effects of High Energy A+A Collisions in RHIC
Hanzhong Zhang
Institute of Particle Physics, Huazhong Normal University, Wuhan
2004年8月4日 威海

2. II. Cross Sections in A+A collisions 1. Hard nucleus model
I. Introduction
II. Cross Sections in A+A collisions
1. Hard nucleus model
2. Modified PDFs
3. Modified FFs
4. NLO cross sections
III. Numerical results and analysis
1. Single hadron production
2. Dihadron production
IV. Conclutions
(Collaborators：Enke Wang, Xin-Nian Wang, Jeff Owens )

3. I. Introduction
Jet Quenching --- Induced by multiple scattering in nuclear medium, the parton jet will radiate gluon and lose energy.
Considering jet quenching effects, Xin–Nian Wang calculated the LO contribution in A+A collisions where a modified factor K=1.5-2 was used to account for higher order corrections.
(Xin-Nian Wang, nucl-th/ )
Here we will investigate the NLO contribution in A+A collisions with jet quenching effects. The modified factor K is not needed.

4. Recent RHIC results suppression of hadron spectrum V2 mono-jet STAR
PHENIX

5. II. Cross Sections in A+A collisions
For hadron production in AA collisions,
the inclusive cross section ∝
thickness function（t） ×
Modified PDFs （f）
Modified FFs（D）
Strong interaction cross sections

6. Single hadron produced in A+A collisions,
Related with t PDF FF pQCD section

7. 1. Hard nucleus model c.m. frame Lorentz contraction. two thin disks.
the nucleons distribute
equally in a static nucleus

8. In reference “X. -N. Wang,Phys
In reference “X.-N. Wang,Phys. Rep 280(1997)287”, the inelastic cross section in AA hard collision is given by
Choose and R=6.5fm
For a given impact parameter, define the centrality

10. 2. Modified PDFs
With the Nuclear Shadowing effects, the modified PDFs are given by
Where the modified factor S is given by HIJING model
(Shi-Yuan Li and X. –N. Wang , PLB527(2002)85)

11. 3. Modified FFs Jet Quenching effects cause to the modification of FF
where
(X. –N. Wang , PRL 68(1996)231, nucl-th/ )

12. 4. pQCD NLO cross sections
LO contribution: all 2→2 subprocesses
NLO contribution:
1) the one-loop corrections added to the 2→2 subprocesses
2) all 2→3 subprocesses
There are soft and collinear singularities in both sets of contributions. Some cancel between the two sets and the remaining ones are factorized and absorbed into the parton distribution and fragmentation functions.

13. 1 . Single hadron production
III. Numerical results and analysis
1 . Single hadron production

14. Pt spectrum
of pi0 meson

15. Nucleus modification factor

16. V2 definition:
Pt distribution of v2
for charged hadron
Data from STAR

17. Invariant mass spectrum
2 . Dihadron production
Invariant mass spectrum
Ratio of AA over pp.

18. The azimuthal distributions

20. v2 of triggled hadron

21. IV. Conclutions
In central A+A collisions, jet quenching suppresses single hadron and dihadron yields in the large Pt (or M) region.
Monojet in central A+A collisions －－ These jets come likely from partons created near the surface of the collision region. The partons that originate or propagate towards the center of the collision region are substantially slowed or completely absorbed(jet quenching)
V2 saturation in large Pt region

22. Thank you very much!
谢谢！

23. Ellipse equation
is given by the ellipse equation

24. Parameterized energy loss
In a finite 1+1 demension expanding medium, the total energy loss is given by (X. –N. Wang , nucl-th/ )
After considering the energy loss detailed balance,
(Enke Wang and Xin-Nian Wang, PRL87(2001)142301)

25. v2 for charged hadron

26. The azimuthal distributions
Here, we assume :
1) compared with the contribution of 2→3 subprocesses, the changed direction caused by jet quenching is little and can be ignored.
2) The azimuthal distributions are only contributed by 2→3 subprocesses